Protective interlayer for high temperature solid electrolyte electrochemical cells

ABSTRACT

A high temperature, solid electrolyte electrochemical cell is made, having a first and second electrode with solid electrolyte between them, where the electrolyte is formed by hot chemical vapor deposition, where a solid, interlayer material, which is electrically conductive, oxygen permeable, and protective of electrode material from hot metal halide vapor attack, is placed between the first electrode and the electrolyte, to protect the first electrode from the hot metal halide vapors during vapor deposition.

GOVERNMENT CONTRACT

The Government of the United States of America has rights in thisinvention pursuant to Contract No. DE-AC-0280-ET-17089 awarded by theU.S. Department of Energy.

This is a division of application Ser. No. 657,923, filed Oct. 5, 1984,now U.S. Pat. No. 4,547,437.

BACKGROUND OF THE INVENTION

High temperature fuel cell generators employing interconnected, tubularfuel cells, with solid electrolytes, are taught by A. O. Isenberg, inU.S. Pat. No. 4,395,468. Fuel electrode, air electrode, solidelectrolyte and interconnection configurations for individual fuelcells, are taught by A. O. Isenberg, in U.S. Ser. No. 323,641, now U.S.Pat. No. 4,490,444 filed on Nov. 20, 1981. Usually, a porous supporttube of calcia stabilized zirconia, approximately 1 millimeter to 2millimeters thick, has an air electrode deposited on it. The airelectrode is from about 50 microns to 1000 microns thick (0.05millimeter to 1 millimeter) and may be made of, for example, LaMnO₃,CaMnO₃, LaNiO₃, LaCoO₃, LaCrO₃, etc. Surrounding the outer periphery ofthe air electrode is a layer of gas-tight solid electrolyte, usuallyyttria stabilized zirconia, approximately 1 micron to 100 microns (0.001millimeter to 0.1 millimeter) thick. A selected radial segment of theair electrode is covered by an interconnect material. The interconnectmaterial may be made of a doped lanthanum chromite film, ofapproximately 50 microns (0.05 millimeter) thickness. The lanthanumchromite is doped with calcium, strontium, or magnesium.

Both the electrolyte and interconnect material are applied on top of theair electrode by a modified chemical vapor deposition process, employingthe use of vaporized halides of zirconium or yttrium for theelectrolyte, or of calcium, magnesium, lanthanum, or the like, for theinterconnect material, at temperatures of up to 1450° C. Such halidevapors can interact with and degrade the air electrode material duringthe initial period of electrolyte and interconnect application. Thiscauses, in some instances, air electrode leaching of dopants, such asstrontium, or leaching of main constituents, such as lanthanum ormanganese. Such leaching causes a resultant, deleterious alteration ofelectrical, chemical, and mechanical properties of the air electrode,due to substantial modification at the electrolyte interface.Additionally, even after electrolyte application, there may be long termdiffusion of manganese from the air electrode into the electrolyteduring operation of the electrochemical cell. There is a need then forsome means to protect the air electrode from highly reactive chlorine orother halide vapors during deposition of the electrolyte andinterconnect layers, and over the long term operations of the cell.

SUMMARY OF THE INVENTION

The above problems have been solved and the above needs met, mostgenerally, by providing a novel doped yttrium chromite, to an interlayerwhich is electrically conductive, permeable to oxygen and protective ofelectrode material, disposed between the electrode and the electrolyte,where, preferably, the layers have an annular structure. Morespecifically, there is provided an oxide interlayer, on top of the airelectrode, which will minimize the degrading of the air electrode fromhot halide vapors, and reduce long term metal diffusion from electrodematerial. This interlayer, preferably, gives a good thermal expansionmatch between itself and the air electrode, electrolyte and interconnectmaterial. It can be sintered onto the air electrode at temperatures ator below vapor deposition temperatures for the electrolyte orinterconnect i.e., 1000° C. to 1600° C., and has good electricalconductivity and oxygen permeability. The most preferred materialmeeting all of these very restricting properties is yttrium chromitedoped with both calcium and cobalt, which has the chemical formula:Y_(1-x) Ca_(x) Cr_(1-y) Co_(y) O₃, where x=from 0.005 to about 0.5 andy=from 0.005 to about 0.5.

This conductive, oxygen permeable, electrode protective interlayer canbe disposed on top of an air electrode in flat or tubular fuel cells ata thickness of from about 0.001 millimeter (1 micron) to about 1millimeter. This interlayer can be applied to the air electrode by anyof a variety of techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made to anembodiment exemplary of the invention, shown in the accompanyingdrawings, in which:

FIG. 1 is an isomeric section view of a singular tubular type fuel cellshowing the interlayer of this invention on top of the air electrode;and

FIG. 2 is a section view through two adjacent fuel cells.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in U.S. Pat. No. 4,395,468, herein incorporated by reference, afuel cell arrangement or stack can comprise a plurality of elongatedannular fuel cells. Each fuel cell is preferably tubular and iselectrically connected at least in series to an adjacent cell. Theelectrical connection is made along a selected axial length of thecells, preferably the entire electrochemically active length. Each cellgenerates an open circuit voltage of approximately one volt, andmultiple cells can be connected in series in order to provide a desiredsystem voltage.

FIG. 1 of the Drawings shows the preferred configuration for the fuelcells of this invention. The preferred configuration is based upon asystem wherein a gaseous fuel, such as hydrogen or carbon monoxide, isdirected axially over the outside of the cell 12, as indicated by thearrow 24, and an oxidant, such as air, or O₂ indicated by the arrow 22,flows through the inside of the cell. It will be recognized that thelocation of the reactant fuel and oxidant can be interchanged such thatair, or O₂ flows about the cells and fuel flows within the cells. This,requires the reversal of the cell electrodes. Where the cell is asshown, oxygen molecules pass through support and air electrode and arechanged to oxygen ions which pass through the electrolyte to combinewith fuel at the fuel electrode. It should be noted that the followingdescription of the prepared tubular configuration should not beconsidered limiting. It should also be noted that the interlayer of thisinvention could be applied to electrochemical cells other than fuelcells, such as oxygen sensors, combustion sensors, electrolysis cells,and the like. The term "air electrode" as used throughout means thatelectrode which will be in contact with oxidant, and "fuel" electrodemeans that electrode that will be in contact with fuel.

In preferred form, each cell 12 includes a porous support tube 26 whichprovides structural integrity to the cell. In an exemplary cell 12, thesupport tube is comprised of calcia stabilized zirconia, forming aporous wall approximately one to two millimeters thick. Surrounding theouter periphery of the support tube 26 is a thin film porous airelectrode, or cathode 27. The exemplary system cathode 27 is a compositeoxide structure approximately 50 microns to 1000 microns (0.05millimeter to 1 millimeter) thick, which is deposited onto the supporttube through well-known techniques. The air cathode is, for example,comprised of doped and undoped oxides or mixtures of oxides, such asLaMnO₃, CaMnO₃, LaNiO₃, LaCoO₃, LaCrO₃, doped indium oxide, In₂ O₃,various noble metals, and other electronically conducting mixed oxidesgenerally composed of rare earth oxides mixed with oxides of cobalt,nickel, copper, iron, chromium and manganese, and combinations of suchoxides. Preferred dopants are strontium, calcium, cobalt, nickel, iron,and tin.

The halide vapor protective, doped yttrium chromite composition, used asthe interlayer of this invention is shown as layer 28 disposed adjacentto and on top of electrode layer 27, forming an interlayer betweenelectrode 27 and solid electrolyte 30, and interconnection material 34.The most preferred interlayer is a calcium and cobalt doped yttriumchromite film, having a preferred thickness of from about 0.001millimeter (1 micron) to about 1 millimeter. This interlayer can beapplied to the air electrode by any of a variety of techniques, such asslurry spraying, dipping, painting, etc. and then sintering, or byplasma-flame-spraying, or physical or chemical vapor deposition. Thepreferred double doped yttrium chromite material has the chemicalformula:

    Y.sub.1-x Ca.sub.x Cr.sub.1-y Co.sub.y O.sub.3,            (I)

where x=from 0.005 to about 0.5, preferably from about 0.05 to about0.2, and y=from 0.005 to about 0.5, y preferably being=from about 0.05to about 0.3. In this particular preferred protective interlayer, bothcalcium and cobalt are present.

Yttrium chromite without any doping elements, while not very reactivewith halide vapors at high temperatures, is not a particularly goodelectrical conductor, and has relatively undesirable thermal expansionproperties. Calcium doped yttrium chromite is a useful protectiveinterlayer material, having fairly good halide vapor protectiveproperties, oxygen permeability and electrical conductivity. However,calcium doped yttrium chromite still has a thermal expansion coefficientlower than that preferred to match the electrolyte, air electrode, andsupport tube. Also adequate sintering of calcium doped yttrium chromite,required during fabrication, is difficult at useful, preferred,fabrication temperatures. Cobalt doped yttrium chromite is also a usefulprotective interlayer material, having fairly good halide vaporprotective properties, oxygen permeability and electrical conductivity,but requires high sintering temperatures. In both calcium or cobaltdoped yttrium chromite, x or y in formula (I) can be zero, i.e., usefulmaterial for the interlayer also includes those materials having thechemical formula:

Y_(1-x) Ca_(x) CrO₃, where x=from 0.0005 to about 0.5, and

YCr_(1-y) Co_(y) O₃, where y=from 0.0005 to about 0.5,

where, of these two, the cobalt composition is preferred.

By adding cobalt to calcium as a dopant, to provide a double dopedyttrium chromite, excellent oxygen permeability is achieved as well asan excellent match of thermal expansion characteristics over the desiredtemperature range of 25° C. to 1000° C. A better sinterability is alsoachieved by using cobalt, as well as an improvement in electricalconductivity i.e., lower resistivity. It is the interaction of bothcalcium and cobalt together, as dopants in yttrium chromite, thatprovides optimum properties and a maximum halide vapor protectiveinterface, minimizing deleterious interactions between halide vapors andthe degrading of the air electrode at temperatures over 1000° C., duringsubsequent vapor deposition of electrolyte and interconnection layers.

The invention should not be considered as limited to the specificpreferred protective interlayer compositions described previously. Theinvention should be considered to include a solid, doped, yttriumchromite material which is electrically conductive, i.e., has aresistivity below about 0.3 ohm-cm at 1000° C., which is oxygenpermeable, and which is protective from hot metal halide vapors whichfrom the solid electrolyte at temperatures over about 1000° C., whichvapors are highly reactive with electrode materials. The interlayershould also approximate the thermal expansion characteristics of theelectrode and electrolyte between which it is disposed, i.e., have anaverage thermal expansion over the range of 25° C. to 1000° C. of fromabout 8×10⁻⁶ M/M°C. to about 13×10⁻⁶ M/M°C. The preferred yttriumchromite materials of this invention are those doped with cobalt andthose doped with both cobalt and calcium.

Generally surrounding the outer periphery of the interlayer 28 is alayer of gas-tight solid electrolyte 30, generally comprised of yttriastabilized zirconia about 1 micron to about 100 microns thick, for theexemplary cell. The electrolyte 30 can be deposited onto the interlayerby well known high temperature vapor deposition techniques. However, aselected radial segment 32 of the interlayer 28 is, for example, maskedduring electrolyte deposition, and a layer of an interconnect material34 is deposited on this segment 32.

The interconnect material 34, which preferably extends the active lengthof each elongated cell 12, must be electrically conductive in both anoxidant and fuel environment. Accordingly, the exemplary cell includes agas-tight interconnection 34 approximately the same thickness as theelectrolyte, about 5 microns to about 100 microns. The preferredinterconnection material is lanthanum chromite doped with calcium,strontium or magnesium.

Substantially surrounding the solid electrolyte 30 is a second porouselectrode, for example, a nickel-zirconia or cobalt zirconia cermet fuelelectrode, as anode 36. As shown the anode 36 is also discontinuous,being spaced from the interconnection 34 a distance sufficient to avoiddirect electrical communication between the anode 36 and both theinterconnection 34 and the cathode 27. The exemplary anode 36 is about100 microns thick.

Deposited over the interconnection 34 is a layer 38 which is preferablycomprised of the same material as the fuel anode 36, nickel or cobaltzirconia cermet, and of the same thickness, about 100 microns.

FIG. 2 shows the series interconnection between consecutive fuel cells12. The electrical interconnection is preferably enhanced by a metalfelt 40, made, for example, of nickel fibers. The felt extends axiallybetween the annular cells 12, and is bonded to each by pressure contactwhich causes sinter bonding during operation. In the inverted cellstructure, where fuel flows inside of the cells, the felt material ismade from conducting oxide fibers, such as doped In₂ O₃ or others.

During operation, air, or O₂ flows through the center of the annularcells 12, and fuel passes over the exterior. Oxygen molecules diffusethrough the porous support 26, cathode 27, and interlayer 28. Fueldiffuses through the anode 36. Oxygen ions pass through the electrolyte30. These reactants electrochemically interact via the actions of theelectrolyte and electrodes in generating products such as water vaporand carbon dioxide, as well as heat and electrical energy. The hightemperature water vapor and carbon dioxide are carried away from thecell with, for example, unburned fuel, and electrical power istransferred in series from the inner cathode 27 of one cell to the outeranode 36 of the next cell. The electrical power is usefully drawnthrough leads not shown.

In the vapor deposition of electrolyte or interconnect materials, metalhalides react with oxygen which diffuses through the growing deposit.This oxygen comes from O₂ or H₂ O gases that are fed into the center ofthe cell, while metal halide vapors surround the outer side of the celltube. Besides the injected metal halides, free chlorine or hydrogenchloride can be produced in the reactions, which take place at or over1000° C. These halide vapors are very reactive and will attack airelectrodes, such as those containing lanthanum, manganese and strontium.The protective interlayer described herein alleviates such degradation,and additional long term diffusion of metal ions, such as manganese,from the air electrode to the electrolyte. It is to be understood thatthe halides also attack the doped yttrium chromite interlayer, however,the resulting reaction products, such as yttrium chloride and chromiumchloride do not interfere with the electrolyte interface in any harmfulway. The doped yttrium chromite is a protective layer, in the sense thatit reacts with the halide vapors instead of the air electrode material,such as doped lanthanum manganite, reacting with the vapors.

In the method of this invention, a porous calcia stabilized zirconiasupport tube, having, for example, a 1.5 millimeter wall and a 13millimeter outside diameter, is covered with 1 millimeter thickness ofair electrode material, for example, doped lanthanum manganite. A 0.5millimeter layer of, for example, calcium and cobalt doped yttriumchromite is then applied, using, for example a slurry sprayingtechnique. The tube containing the double doped yttrium chromite layersis then heated in air at about 1200° C. to 1600° C. for about 3 hours to1 hour, to form a sintered chromite layer integrally bonded to the airelectrode. The chromite layer is then masked over the radial segmentwhere the interconnect is to be deposited later. The electrolyte is thenapplied by vapor deposition of metal oxides from gaseous YCl₃ and ZrCl₄,at about 1200° C. After demasking the radial segment, the interconnectmaterial is applied over the doped yttrium chromite layer by vapordeposition, using chloride vapors of chromium, lanthanum, and magnesium.Finally the fuel electrode is applied over the electrolyte. Here thedouble doped yttrium chromite acts as a sacrificial, halide vaporprotective interlayer between the air electrode, the interconnection,and electrolyte materials during their deposition at high temperatures.

EXAMPLE 1

To investigate the bulk properties of various intermediate layer oxides,the component oxides were ground, mixed, pressed in a steel die, atabout 5,000 psi, and then sintered on platinum foil in an oven at from1300° C. to 1600° C., to form 1"×0.25"×0.25" bars having samplecompositions 1 through 6 described further in Table 1. The density wasdetermined, four terminal resistance measurements taken, and thermalexpansion measured using a dilatometer method. The results are shownbelow in Table 1 where Sample 6 is a support tube sample:

                                      TABLE 1                                     __________________________________________________________________________                                        Average Thermal                                                               Expansion                                               Heat Treatment                                                                          Calculated                                                                          Resistivity                                                                         in M/M° C.                         Sintered Sample                                                                             Temp., Time, and                                                                        Density                                                                             ohm-cm                                                                              over range                                Composition   Gas Atmosphere                                                                          grams/cm.sup.3                                                                      at 1000° C.                                                                  25° C. to 1000°             __________________________________________________________________________                                        C.                                        (1) Y.sub.0.9 Ca.sub.0.1 CrO.sub.3                                                          1400° C. 50 min. Air                                                             3.1   0.270  8.1 × 10.sup.-6                                  1600° C. 2 hr. N.sub.2                                                           4.1                                                                 1300° C. 18 hr. Air                                      (2) Y.sub.0.9 Ca.sub.0.1 Cr.sub.0.8 Co.sub.0.1 O.sub.3                                      1450° C. 2 hr. Air                                                               4.7   0.043  8.2 × 10.sup.-6                    (3) Y.sub.0.9 Ca.sub.0.1 Cr.sub.0.85 Co.sub.0.15 O.sub.3                                    1500° C. 3 hr. Air                                                               5.5   0.032 10.6 × 10.sup.-6                    (4) Y.sub.0.9 Ca.sub.0.1 Cr.sub.0.8 Co.sub.0.2 O.sub.3                                      1400° C. 1 hr. Air                                                               5.4   0.031 12.2 × 10.sup.-6                    (5) YCr.sub.0.8 Co.sub.0.2 O.sub.3                                                          1600° C. 1 hr. Air                                                               5.5   0.061                                           (6) (ZrO).sub.0.85 (CaO).sub.0.15   10.0 × 10.sup.                      __________________________________________________________________________                                        -6                                    

As can be seen, Sample 1 (no cobalt) has a much lower thermal expansionthan Sample 6 (a typical support tube material for a high temperaturefuel cell) and a low density. Resistivity is also relatively high.Adequate sintering was found to take a relatively long time period.Preferred double doped Samples 2 through 4 provided excellent lowresistivity values, and high densities, along with good temperature-timesintering parameters. Sample 3 showed excellent thermal expansionmatching characteristics to the support tube sample 6. Sample 5 (nocalcium) showed relatively high resistivity values, less conductivitythan Samples 2 through 4, good density but relatively high sinteringtemperatures. All of the Samples 1 through 5 show good oxygen moleculepermeability and are considered useful interlayer materials.

EXAMPLE 2

An interlayer having the composition of Sample 1 of TABLE 1 of EXAMPLE 1and a thickness of about 0.025 millimeter was slurry spray depositedonto a doped lanthanum manganite air electrode which was deposited ontoa calcia stabilized zirconia support tube. The 30% porous support had a13 millimeter outside diameter and was covered with the La₀.9 Sr₀.1 MnO₃air electrode, which was about 1 millimeter thick. This layered tube wassintered at 1400° C. for about 1 hour. Yttrium stabilized zirconiaelectrolyte was vapor deposited onto the Y₀.9 Ca₀.1 CrO₃ layer at about1200° C., in the form of halide vapors, followed by fuel electrodeapplication, to provide a tubular fuel cell. This fuel cell was comparedfor stability at 1000° C. with a similar fuel cell using no chromiteinterlayer between the air electrode and the interconnect orelectrolyte. The fuel cell having the Y₀.9 Ca₀.1 CrO₃ interlayer showedbetter performance and stability at operating conditions, attributableto less air electrode attack during electrolyte vapor deposition. Oxygenpermeability was not inhibited by the presence of the interlayer.Interlayers containing cobalt and calcium dopants, as in Samples 2through 4 of the TABLE of EXAMPLE 1, would provide even better operationof the fuel cell over long operating times.

We claim:
 1. A method of making a high temperature, solid electrolyteelectrochemical cell comprising the steps:(1) providing a poroussupport, (2) applying a first electrode, which is subject to degradationby hot metal halide vapors, to the support, (3) applying an electricallyconductive, oxygen permeable interlayer material on the first electrode,to protect the first electrode from degradation by hot metal halidevapors, (4) contacting the interlayer material with hot metal halidevapors, to form a metal oxide solid electrolyte layer over theinterlayer material, and (5) applying a second electrode over the solidelectrolyte.
 2. The method of claim 1, where the interlayer material isselected from the group consisting of calcium and cobalt doped yttriumchromite, calcium doped yttrium chromite, the cobalt doped yttriumchromite.
 3. The method of claim 1, where the interlayer material hasthe chemical formula Y_(1-x) Ca_(x) Cr_(1-y) Co_(y) O₃, where, x=from0.005 to about 0.5 y=from 0.005 to about 0.5.
 4. The method of claim 1,where the support is a tube comprised of calcia stabilized zirconia, thefirst electrode is comprised of doped and undoped oxides or mixtures ofoxides, the solid electrolyte is comprised of stabilized zirconia, andthe second electrode is selected from the group consisting of nickelzirconia cermet and cobalt zirconia cermet.